The present invention relates to a display device being able to be controlled the intensity of light emitted from, e.g., an electro-luminescent (EL) element or a light emitting diode (LED) to illuminate at a desired level of luminance. More particularly, the present invention relates to a display device having a matrix of light emitting elements as the pixels and having uniform emission of light from the pixels without variations, and thus displaying a highly elaborate image with definite steps of gradation.
With development of the office automated (OA) instruments, their: displays are increasingly demanded for lowering the power consumption and minimizing the overall size. Common examples of such conventional display devices are CRTs and liquid crystal display panels. Recently, other display devices are focused including a large-screen LED system installed at a corner of a town or a thin, small self-illumination device using EL elements as the display on an OA instrument.
In a conventional CRT, the emission (luminance) of light of each pixel can be changed by controlling the intensity of electron beam but fails to be automatically controlled in response to variations of the phosphoric screen. Also, in a liquid crystal display, the brightness of each pixel can be changed by controlling the duty ratio for duty drive. It however fails to automatically control the brightness in response variations of the color filter.
A self-illumination display device having above mentioned light emitting element, for example an LED or an EL element, functions by driving each pixel with a drive voltage (or current) corresponding to a certain level of input power.
As described above, any conventional display device is designed to be driven by a uniform level of power unless a specific mode of display such as gradations is desired. However, the self-illumination display device using light emitting elements such as LEDs or ELs has their pixels provided independent from each other and may generate non uniformity of their emission. When such light emitting elements which are not uniform in the luminance of emitting light are used in a matrix form as a display, they may yield blurs and noises hence reproducing an image of unfavorable quality. In particular, desirable steps of gradation may hardly be reproduced by a given input power if the pixels emit variations of the luminance.
In reverse, the self-illumination display device using light emitting elements such as LEDs or ELs may favorably be changed in the luminance by controlling the applying power which is higher than a particular threshold level of voltage (or current). Accordingly, when a variation in the illumination is identified, it may adjustably be suppressed to render the illumination uniform.
The present invention is developed in view of the above aspects and its object is to provide a display device having each light emitting element adjusted to emit a desired level of illumination determined by the input level regardless of its illumination characteristics.
It is another object of the present invention to provide a display device which has a matrix of light emitting elements provided as pixels and is adjusted for controlling the illumination of each pixel thus to display, an image which hardly has blurs or noises and appears with highly definite steps of gradation.
A display device according to the present invention is provided comprising a light emitting element, a light receiving element for monitoring the amount of light emitted from the light emitting element, and a control circuit for controlling the amount of light emitted from the light emitting element according to an output of the light receiving element, whereby the amount of light emitted from the light emitting element can favorably be adjusted.
This allows the light emitting element to be controlled by the action of the control circuit so that an output of the light emitting element corresponds to the input signal (of a given voltage level) irrespective of characteristics of the light emitting element when the light emitting element fails to emit a desired level of light output (luminance).
The control circuit may comprise a comparator for comparing between a voltage of the output of the light receiving element and a voltage of the input signal and a driver MOS transistor for driving the light emitting element, wherein an output of the comparator as the driver signal to the gate of the driver MOS transistor being fed for determining a driving voltage to the light emitting element.
It may be modified in which a plurality of pixels are arrayed in a matrix form, each pixels comprising a group of the light emitting element, the light receiving element, and the drive MOS transistor, the gate of the driver MOS transistor of each pixel being connected to a capacitor for holding a gate voltage of the driver MOS transistor, each pixel also including a control terminal for controlling simultaneously the action of a first switching element for switching on and off the driver signal fed to the gate of the driver MOS transistor and the action of a second switching element for switching on and off the output of the light receiving element fed to the comparator circuit, whereby the light amount of the light emitting element of each pixel can be adjusted by the selecting action of the control terminal. Accordingly, the illumination of the light emitting elements arrayed in a matrix form can be controlled at each pixel respectively.
The driver MOS transistor and the light emitting element may be connected in series between the power source voltage and the ground so that the driver power to the light emitting element can be adjusted by the driver signal fed to the gate of the driver MOS transistor. The illumination of each of the light emitting elements can thus be controlled using a simple construction.
It may also be modified in which while the first switching element of each of the pixels aligned in a column or a row is connected at one end to a driver line, the second switching element of each of the pixels alligned in a column or a row is connected at one end to a monitor line, and the control terminal of each of the pixels aligned in a row or a column is connected to a select line, a first shift register is provided for sequentially scanning the columns or rows of the driver line and the monitor line and a second shift register is provided for sequentially scanning the rows or columns of the select line. Accordingly, while each pixel is selected by scanning the column and the row, its illumination can be controlled to have an image of equality.
It may further be modified in which a plurality of groups are arrayed in a matrix form in a semiconductor substrate, each of the groups constituting said pixel, and comprising the light receiving element, the driver MOS transistor, the capacitor, the first switching element, and the second switching element, and a plurality of light emitting elements, each of which is said light emitting element, are arrayed in a matrix form the light emitting element provided as associated with the each group on the semiconductor substrate and arranged for emitting light from upper and lower surfaces. Accordingly, the display device can be simple in the construction and improved in the high imaging concentration with a minimum of non-illumination area.